Lithography is utilized in semiconductor device manufacturing to pattern features on semiconductor workpiece layers for integrated circuit fabrication.
FIG. 1 shows a lithographic fabrication system 100 for defining features in a workpiece 120, in accordance with prior art. Typically, workpiece 120 comprises a semiconductor substrate, together with one or more layers of substances (not shown) such as silicon dioxide and a resist layer 101, affixed to a surface of workpiece 120.
Typically, radiation of wavelength λ is emitted by an optical source 106, such as a mercury lamp or a laser. The radiation propagates through an optical collimating lens or lens system 104, a patterned lithographic mask 103 having a pattern of opaque and transparent features, and an optical projection lens or lens system 102. The radiation transmitted through mask 103 is imaged by lens 102 onto resist layer 101, thereby exposing a patterned area corresponding to the mask pattern. If resist layer 101 is positive, exposed areas will be subject to removal after development and if it is negative, exposed areas will remain intact. Thus, the pattern of mask 103 is transferred to (“printed on”) resist layer 101. “Mask” as used herein means “mask” and/or “reticle”.
As known in the prior art, the indicated distances L1 and L2 satisfy, in cases of a simple lens 102, 1/L1+1/L2=1/F, where F is the focal length of lens 102. A pattern produced by mask 103 on resist layer 101 will be substantially in focus if resist layer 101 is a distance L2 from projection lens 102. This conclusion is based on a geometrical optics analysis which assumes light travels in straight lines. However, when the feature size is comparable to λ/NA, where λ is the illumination wavelength, and NA is the numerical aperture of the projection lens, a physical optics analysis should be considered which includes the wave nature of light. Under this analysis diffraction effects are likely to be produced, decreasing the image resolution even at distance L2, thereby reducing resolution of component features. For semiconductor devices it is desirable to maximize the number of circuit components per unit area by minimizing component size. As component size decreases, diffraction effects become more significant, thereby limiting reduction in component size. Decreased sharpness of mask images caused by diffraction effects may reduce product yield and increase device failure rate.
Diffraction effects may be severe for conventional or binary masks. FIG. 2A depicts a cross-sectional view of a prior art binary mask 10. Binary mask 10 typically comprises a glass or quartz layer 12 with a patterned chromium layer 40 affixed thereto. The patterned chromium layer comprises a plurality of substantially transparent areas 14, 15 and 16 and a plurality of attenuating areas 18, 19, 20 and 21. Electromagnetic radiation propagating through areas 14, 15 and 16 have electric fields associated therewith. Amplitudes of the electric fields at the mask level are represented with respect to a cross-section of the mask in FIG. 2B, wherein steps 36, 37 and 38 correspond to electric fields from radiation propagating through apertures 14, 15 and 16, respectively. Because of the wave-nature of the radiation it spreads as it propagates. Therefore, even though the electric fields are separated from one another at mask level they may interfere with one another a distance away from the mask, such as at a workpiece surface. This is shown in FIG. 2C. Due to the diffraction effect, it is clear that the electric field at the workpiece surface spreads wider relative to that at the mask level. The smaller the feature sizes, as represented by transparent areas 14, 15, and 16, the wider the spread.
Solid lines 22 and 24 in FIG. 2C represent electric fields from apertures 14 and 16, respectively, and broken Line 26 represents an electric field from aperture 15. The amplitudes of the electric fields from adjacent openings (14 and 15, for example) overlap in cross-hatched regions 30 and 32. As shown in FIG. 2D, this interference or constructive addition of electric field amplitudes results in an electric field 34 which has a higher intensity at the workpiece surface in regions 30 and 32, relative to the surrounding areas than at mask level. Therefore, there is less contrast in the light intensity distribution at the workpiece surface than at mask level, thereby reducing the resolution capability of the tool.
Undesirable diffraction effects become more significant with small dimension pattern features. “Small dimension” as used herein means small size and small spacing between transparent regions relative to λ/NA, where λ is the wavelength of the optical source and NA is the numerical aperture of the projection system.
It is known in the art to improve the system resolution by employing phase-shifting masks. The mask imparts a phase-shift to the incident radiation, typically by π radians. Phase-shifting masks generally comprise transparent areas having an optical intensity transmission coefficient T, near 1.0 at the incident radiation wavelength λ, attenuating areas or partially transparent areas having T at λ in the range of about 0.05 to about 0.15, and, optionally, opaque areas, having T less than or equal to about 0.01.
FIG. 3A depicts a cross-sectional view of a prior art π radian-phase-shifting mask 300. Mask 300 is substantially similar to binary mask 10 but includes a phase-shifter layer 310 over transparent regions 14 and 16. Phase-shifter layer 310 reverses the direction of the electric field vectors at apertures 14 and 16 relative to aperture 15 as shown in FIG. 3B at 320, 322 and 330. The π radian phase-shift is created by employing a phase-shifter layer 310 with a thickness of d=λ/2(n−1) where λ is the wavelength of the optical source and n is the refractive index of layer 310 at λ. The phase-shifter layer modifies the optical distance traveled by incident radiation, thereby producing a phase-shift. As is shown in FIG. 3C, by peaks 340, 345 and 350, the overlapping regions of adjacent electric fields have opposite amplitudes, and therefore, a destructive interference occurs. The cancellation of the electric field at those locations improves the contrast of the intensity field as shown in FIG. 3D. FIG. 3E depicts a vector diagram of the electric field at a workpiece level produced by radiation propagating through a n radian-phase-shifting mask. Vector 380 represents an electric field from unshifted radiation such as passes through aperture 15. Vector 390 corresponds to phase-shifted radiation such as that which propagates through aperture 14 and phase-shifter 310. The amplitude of vector 390 equals the negative of the amplitude of vector 380, thereby canceling it out upon interference.
Phase-shifting masks producing π radian shifts are an improvement over binary masks. However, they do not fully resolve all resolution problems, for example a phase conflict may arise for feature configurations in which a phase transition is generally unavoidable. Whenever a phase transition occurs a dark line will result.
Electric field interference has been addressed by using a mask having a π/2 radian shift and a 3/2π radian shift. Liebmann et al, “Alternating Phase Shifted Mask for Logic Gate Levels, Design and Mask Manufacturing”, SPIE vol. 3679 p. 27 (1999). It is also known in the art to use π:⅔ π: ⅓ π:0 radian shifting masks.
It is therefore desirable to reduce phase conflict thereby substantially eliminating undesirable lines, and thus facilitating feature size reduction and improving product yield and reliability.